    Constant Current Sources A True Constant Current Source: The WT2 'Low Power' Test Port The WT2's propriatary 'low power' test port circuit delivers a true constant current to the test terminals. Though the voltage is limited to the USB supply rail (+/-2V from center bias of 2V), the output appears to be a true constant current with an output impedance in the megohm range. In this case Vout is directly proportional to the load impedance. An interesting advantage of the constant current source high impedance is the ability to measure high impedance devices. An example would be the ability to accurately measure capacitors as small as 100 pF. A special capacitor measurement mode setup is used to provide a DC return path and null out any residual parasitic capacitance. This FAQ discussing measurement of inductors, resistors and capacitors may also be of interest. Approximated (Faux) Constant Current The circuit shown here has been used in the the speaker industry for many years to simulate a constant current. By applying a voltage divider equation, it can then be seen that as long as the device impedance is relatively small, the bulk of the applied voltage appears across the series resistor, and the current is more or less constant. This assumption however begins to quickly fall apart when the ratio is less then 10:1. Impedance peaks of several hundred ohms are quite common in modern loudspeakers making high value series resistors desirable. Additionally the driving amplifier needs to have sufficient drive voltage to achieve a usefull signal level. Even the 1k series resistor that is shown is a little too small (but it does make the math easier). Using a simple test jig of say a 100 ohm series resistor hung on the output of a PC sound card might be convenient, but it provides neither constant current or constant voltage. Constant Voltage (not very common) Completing the measurement circuit topologies is constant voltage. In this case a low value current sensing resistor is placed inline with the circuit. The voltages across the load and sensing resistor are then differentially amplified and measured. The math is similar to that of the faux constant current source, except that the ratio is now inverted. And once again, it would be desirable to have a 10:1 ratio. The advantage is that achieving high drive level is relatively easy but at the expense of not being constant current. Another significant disadvantage is that a shorted test load will produce a very high current. WTPro Hi-ZP Port - Achiving Constant Current Using Software One of the selectable WTPro methods uses a software feedback loop to produce the same effect as a true constant current source.  Initially Z is measured, and then V1 is adjusted knowing I=V/Z, the result being a constant current.  This itterative process takes several loops to narrow down V1, and the resulting I, to an acceptable tolerance. A much faster WTPro method when measuring driver TS parameters is to first find the Fs impedance peak.  The 'Zo points' that define the Q are a function of Re and Zmax, allowing Zo to be precalculated, and from that V1 can be pre-adjusted.   Again, the result is that of a constant current but without needing a software feedback loop.  In other words, this produces the same results but is much faster. The remaining WTPro options include simple voltage (no feedback at all), or constant voltage using feedback.